It's not about the money, either. LetsEncrypt had their intermediate certificates cross-signed by IdenTrust, otherwise they wouldn't have the 30% market share.
It takes a lot of time with the CA application process. You will need to convince Microsoft, Apple, Mozilla, Java, etc that you will be a good CA with good practices. Multiple security audits (starting at about $40K... KPMG don't come cheap), hardware (HSM, servers for validation, OCSP, CT, etc) and people (needs to be trusted people, and often are paid top dollars).
It's easy to _buy_ a CA than creating one. You can get your new root certificates included later easily now that you have a root already, from which you can cross-sign until they are included. CAs are sold out more often than many of us think.